1. Field of the Invention
The present invention relates to a solar cell of high efficiency, a method of producing the same and an apparatus for producing a semiconductor.
2. Discussion of Background
In order to obtain a silicon solar cell of high efficiency it is essential to form minute projections and recesses in a substrate surface so that incident light entering from the substrate surface can be efficiently taken inside the solar cell. Various methods have been proposed to realize such solar cell.
In a solar cell of single crystal silicon, minute pyramidal projections and recesses called a texture structure are formed by a wet etching method using an aqueous alkali solution such as caustic soda, caustic potash or the like. Because the texture structure is constituted by many inclined planes, light reflected at a surface hits a surface of another position and re-enters into the inside whereby the light is absorbed efficiently in the solar cell. A part of the incident light reaches the rear surface without being absorbed in the substrate. Such light is reflected at the rear surface to reach again the front surface where the texture structure is formed. Accordingly, the light is reflected again at an inclined plane and confined in the solar cell. Thus, a substantial part of light is absorbed in the solar cell to thereby improve the characteristics of the solar cell. However, this technique utilizes a difference of etching rate at crystal planes in a silicon crystal. Namely, the etching rate obtained when using an aqueous alkali solution is fastest at a (100) plane of silicon and slowest at a (111) plane. Accordingly, when etching is initially effected to a (100) plane and if a (111) plane is subsequently produced due to any incident during the etching process, the (111) plane delays the etching rate and remains in the surface as a dominant plane.
Since the (111) plane has an inclination of about 54 degree to the (100) plane, pyramidal projections each constituted by only (111), (111) and (111) planes which are equivalent to the (111) plane are formed at the final stage of the process. The process will be described in more detail below. A substrate of silicon having a (100) plane at its surface is dipped in an aqueous solution of caustic potash or caustic soda having a concentration of from several % to 10 and several % heated to 60.degree. C. to 95.degree. C. for 10 min to 30 min. In some cases, isopropyl alcohol of 5% to 30% by volume per an aqueous alkali solution may be added. After the dipping, the substrate is taken out and is washed with water.
In a case of using a substrate of polycrystal silicon which has a variety of directions of crystal in a plane, a pyramidal structure is formed perpendicular to the (100) plane. Accordingly, planes appearing in the front surface are in random directions, and a sufficient light confining effect as obtained in the substrate of single crystal cannot be expected. Further, when the etching liquid having the above-mentioned composition is used, the depth of etching varies depending on the plane directions exposed at the front surface. Thus, steps which do not provide an antireflection effect and prohibits formation of effective electrodes are produced.
Accordingly, various techniques of forming the texture structure have been studied for the substrate of polycrystal silicon, other than the etching method using an aqueous alkali solution for the silicon substrate of single crystal. Further, the conventional method requires a long time for the treatment and is not high in productivity. For example, the conventional method requires a treating time of about 30 min in order to form stably a texture structure in the substrate of single crystal silicon dipped in an etching liquid prepared by adding isopropyl alcohol of 30% by volume to a 1% aqueous potassium hydroxide solution, which is heated to 90.degree. C.
As a first example, there is proposed a method disclosed in Japanese publication JP-B-7-105518 wherein a projection/recess structure is formed by forming mechanically V-like grooves in the front surface of a solar cell of polycrystal silicon. FIG. 13 shows in cross section the structure formed in this example.
As a second example, there is proposed a method disclosed in 9th International Photovoltaic Science and Engineering held on 11 to 15, November, 1996 wherein a pyramidal structure is formed in the front surface of a solar cell of polycrystal silicon by an etching method called RIE (Reactive Ion Etching). FIG. 14 shows a microphotograph of a projection/recess structure formed by RIE.
The detail of the above-mentioned examples will be described.
In the method of mechanically forming the V-like grooves in the first example, a plurality of rotating blades in which material having a hardness higher than silicon, such as diamond, silicon carbide or the like is embedded, are pushed to the silicon substrate, and the rotating blades are dragged on the substrate to thereby form the V-like grooves in the surface of the substrate. The pitch of the V-like grooves is generally in a range of from several hundred .mu.m to several mm, which is adjustable by adjusting the distance of the blades. The depth of the V-like grooves is generally from several ten .mu.m to 100 .mu.m. After mechanically forming the grooves, the substrate is dipped in a solution such as an aqueous alkali solution or the like which is capable of etching silicon whereby a defective crystal layer produced in an area to where the blades have been in mechanical contact, is removed.
In the method of forming projections and recesses by RIE as in the second example, a chlorine gas is used as an etching gas, and silicon is reacted with chlorine ions and chlorine radicals produced by plasma under a reduced pressure to form a chloride of silicon so that the silicon is removed by evaporation. Although the mechanism of forming the projection/recess structure is unclear because the publication fails to disclose it, it is supposed that a part of the silicon chloride as a reaction product remains in the front surface because the etching is conducted without using an etching mask, and cylindrical projections are formed by utilizing the reaction product as a micro-mask. After the formation of the projection/recess structure in the front surface, the substrate is subjected to a wet type cleaning so that the reaction product remaining on the front surface is removed. Thus, a series of treatments is finished.
The conventional method of forming mechanically the V-like grooves requires grinding treatment for each wafer, and presents a problem in large scale production. Further, the conventional techniques requires a step of removing the defective layer by the wet type etching because there was a defect in the surface portion of crystal when the V-like grooves were formed. In order to obtain effectively a light confining effect, it is necessary to form deep V-like grooves. However, the grooves cannot be formed with a small distance because the pitch of the V-like grooves is restricted by the pich of the rotating blades. On the other hand, the substrate has to be thin in order to reduce the cost of material. Accordingly, a crack may be produced in the substrate, or breakage may be caused during manufacturing steps when deep grooves are formed in the substrate.
In the method by using RIE, the problem of producing a defective crystal as in the case of forming the V-like grooves can be avoided. However, there are problems such as a high manufacturing cost due to use of a vacuum device and a poor productivity due to a small performance of treatment.